1. Field of the Invention
The present invention relates generally to buoyancy systems for offshore oil platforms. More particularly, the present invention relates to a buoyancy system with an internal beam.
2. Related Art
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from less productive oil reserves, or to reach more distant oil reserves. Such distant oil reserves may be located below the oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves. In addition, some oil reserves are located farther offshore, and thousands of feet below the surface of the oceans.
For example, vast oil reservoirs have recently been discovered in very deep waters around the world, principally in the Gulf of Mexico, Brazil and West Africa. Water depths for these discoveries range from 1500 to nearly 10,000 ft. Conventional offshore oil production methods using a fixed truss type platform are not suitable for these water depths. These platforms become dynamically active (flexible) in these water depths. Stiffening them to avoid excessive and damaging dynamic responses to wave forces is prohibitively expensive.
Deep-water oil and gas production has thus turned to new technologies based on floating production systems. These systems come in several forms, but all of them rely on buoyancy for support and some form of a mooring system for lateral restraint against the environmental forces of wind, waves and current.
These floating production systems (FPS) sometimes are used for drilling as well as production. They are also sometimes used for storing oil for offloading to a tanker. This is most common in Brazil and West Africa, but not in Gulf of Mexico as of yet. In the Gulf of Mexico, oil and gas are exported through pipelines to shore.
Certain floating oil platforms, known as spars or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend thousands of feet to reach the ocean floor, and the oil reserves beyond.
Typical risers are either vertical (or nearly vertical) pipes held up at the surface by tensioning devices (called Top Tensioned riser); or flexible pipes which are supported at the top and formed in a modified catenary shape to the sea bed; or steel pipe which is also supported at the top and configured in a catenary to the sea bed (Steel Catenary Risers—commonly known as SCRs).
The flexible and SCR type risers may in most cases be directly attached to the floating vessel. Their catenary shapes allow them to comply with the motions of the FPS caused by environmental forces. These motions can be as much as 10–20% of the water depth horizontally, and 10s of feet vertically, depending on the type of vessel, mooring and location.
Top Tensioned risers (TTRs) typically need to have higher tensions than the flexible risers, and the vertical motions of the vessel need to be isolated from the risers. TTRs have significant advantages for production over the other forms of risers, however, because they allow the wells to be drilled directly from the FPS, avoiding an expensive separate floating drilling rig. Also, wellhead control valves placed on board the FPS allow for the wells to be maintained from the FPS. Flexible and SCR type production risers require the wellhead control valves to be placed on the seabed where access is difficult and maintenance is expensive. These surface wellhead and subsurface wellhead systems are commonly referred to as “Dry tree” and “Wet Tree” types of production systems, respectively. Drilling risers must be of the TTR type to allow for drill pipe rotation within the riser. Export risers may be of either type.
TTR tensioning systems are a technical challenge, especially in very deep water where the required top tensions can be 1,000,000 lbs (1000 kips) or more. Some types of FPS vessels, e.g. ship shaped hulls, have extreme motions which are too large for TTRs. These types of vessels are only suitable for flexible risers. Other, low heave (vertical motion), FPS designs are suitable for TTRs. This includes Tension Leg Platforms (TLP), Semi-submersibles and Spars, all of which are in service today.
Of these, only the TLP and Spar platforms use TTR production risers. Semi-submersibles use TTRs for drilling risers, but these must be disconnected in extreme weather. Production risers need to be designed to remain connected to the seabed in extreme events, typically the 100-year return period storm. Only very stable vessels, such as TLPs and Spars are suitable for this.
Early TTR designs employed on semi-submersibles and TLPs used active hydraulic tensioners to support the risers by keeping the tension relatively constant during wave motions. As tensions and stroke requirements grow, these active tensioners become prohibitively expensive. They also require a large deck area, and the loads have to be carried by the FPS structure.
Spar type platforms recently used in the Gulf of Mexico use a passive means for tensioning the risers. These type platforms have a very deep draft with a central shaft, or centerwell, through which the risers pass. Types of spars include the Caisson Spar (cylindrical), the “Truss” spar and “Tube” spar. There may be as many as 40 production risers passing through a single centerwell.
It will be appreciated that these risers, formed of thousands of feet of steel pipe, have a substantial weight, which are supported by buoyant elements at the top of the risers. Steel buoyancy cans (i.e. air cans) have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. The steel buoyancy cans are typically cylindrical, and they are separated from each other by a rectangular grid structure referred to as riser “guides”.
These guides are attached to the hull. As the hull moves, the tops of the risers are deflected horizontally with the guides. However, the risers are tied to the sea floor and have a fixed length; hence as the vessel moves horizontally the risers slide up and down (from the viewpoint of a person on the vessel the risers are moving vertically within the guides).
A wellhead at the sea floor connects the well casing (buried below the sea floor) to the riser with a special Tieback Connector. The riser, typically 9–14 inch diameter pipe, passes from the tieback connector through thousands of feet of seawater to the bottom of the spar and into the centerwell. Inside the centerwell the riser passes through a stem pipe, or conduit, which goes through the center of the buoyancy cans. This stem extends above the buoyancy cans themselves and supports the platform to which the riser and the surface wellhead are attached. The stem can be centered in the buoyancy cans by a “wagon wheel” type frame or spacer to hold or centralize the stem within the can. The riser can be centered in the stem by a “wagon wheel” type frame or spacer to hold or centralize the riser within the stem.
Since the surface wellhead (“dry tree”) move up and down relative to the vessel, flexible jumper lines connect the wellhead to a manifold which carries the oil to a processing facility to separate water, oil and gas from the well stream.
The underlying principal of the buoyancy cans is to remove a load-bearing connection between the floating vessel and the risers. The buoyancy cans need to provide enough buoyancy to support the required top tension in the risers, the weight of the cans and stem, and the weight of the surface wellhead. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture.
In addition, as risers have become longer by going into deeper water, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the available width and length of the well bays within the platform structure. Thus, when additional buoyancy has been required, the natural solution has been to extend the length or number of the air cans. One disadvantage with more and/or larger air cans is that the additional length air cans adds more and more weight which also must be supported by the air cans, decreasing the air can s ability to support the risers. Another disadvantage of simply stringing more air cans together is that their weight and length make it very expensive, technically difficult and dangerous to install the buoyancy cans into the vessel's centerwell. Some of these steel air cans are up to 400 feet long and weigh 160,000 lbs. Another disadvantage with merely stringing a number air cans is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
In addition to providing buoyancy, the air cans also are subjected to loads or forces between the air can and the vessel. For example, the air cans are also subjected to side loads and bending loads caused by hydrodynamic loads acting on the air cans during vessel movement. Thus, air cans usually must be designed to address both buoyancy and dynamic loading.